Field of the Invention
The present invention relates generally to methods and apparatus for processing interference due to non-linear products of transmitted signals in a wireless network, and more specifically, but not exclusively, to detection and reduction of interference caused to a receiver due to passive intermodulation (PIM) products.
Description of the Related Technology
Non-linear products may be generated in a wireless network when one or more signals are transmitted along a signal path including a component having a non-linear transmission characteristic; these products may differ in frequency from the signal or signals from which they were generated, and may potentially cause interference to other signals. The generation of non-linear products is becoming a problem of increasing importance in modern wireless communication systems, and in particular cellular wireless systems, since the radio frequency spectrum available has been steadily expanded as additional bands have become available, and the pattern of allocation of uplink and downlink bands within the available spectrum for use by various cellular systems, such systems using GERAN (GSM EDGE Radio Access Network), UTRAN (UMTS Terrestrial Radio Access Network) and E-UTRAN (Evolved UMTS Terrestrial Radio Access Network) radio access networks, and by various operators, is complex and territorially dependent. In this environment, and in particular at a cellular radio base station, it is likely that circumstances arise in which non-linear products generated from transmitted carriers in one or more downlink bands would fall within an uplink band in which signals are received at the base station. Non-linear products generated by non-linear characteristics of active components such as power amplifiers may generally be dealt with at a design stage by appropriate frequency management and filtering, but non-linear products caused by non-linear characteristics of passive components, for example passive intermodulation (PIM) products, may prove more difficult to manage. Many passive components may exhibit a non-linear transmission characteristic to some degree, for example due to an oxide layer at a metal to metal contact, and the non-linear characteristic may develop with time as an ageing process of the component. Furthermore, PIM may be caused by components in the signal path between the transmitter and the receiver that are external to the transceiver equipment and which may be outside the operator's control, such as metallic objects on the antenna tower or other objects in the signal path through the propagation environment, such as fences; this is known as the “rusty bolt” effect.
Interference due to PIM may reduce the carrier to interference ratio at a receiver, which may reduce coverage by a cell significantly. Conventionally, as a solution to the problem of suspected interference caused by passive non-linear products in a cellular radio network, the source of the passive non-linear products may be tracked down by field engineers and a component causing the interference may be replaced. However, this is labor intensive and relies on the problem being identified in order to alert the field engineers.
Methods and apparatus for reduction and detection of intermodulation products have been disclosed in published in the applicant's GB patent applications GB2502279 and GB2502281. Simulated intermodulation products are generated and used to detect or reduce intermodulation products in a received signal.
It is an object of the invention to address improvements to reduction and detection of intermodulation products.